Scaling mongodb requires changes to all mongo confs as well as broker configs. Scaling activemq requires changes to all broker and node confs. Is a heat template able to propagate changes widely like that? Alternatively we'd have to implement some kind of discovery mechanism...

BTW, this will need to be easily customizable to either plug into existing DDNS infrastructure, or provide HA BIND (not everyone will have a workable DDNS solution handy).